1. Field of the Invention
The present invention generally relates to vehicle battery temperature control. More specifically, the present invention relates to a vehicle battery temperature control system and method that use liquid phase and vapor phase coolant to maintain desired battery temperature, and add thermal energy to the coolant to increase the coolant temperature as desired.
2. Background Information
A hybrid electric vehicle (HEV) or full electric vehicle relies substantially or completely on battery power for operation. Therefore, it is desirable to maintain the battery cells at an optimal operating temperature. As understood in the art, battery cells are often best suited to operate in a somewhat small optimum temperature range. Consequently, the life or durability of the battery, as well as the performance of the battery, such as the available power provided by the battery, can be adversely affected by temperatures above and below that optimum temperature range. Furthermore, to optimize battery performance, it is essential to maintain temperature uniformity among the cells of a battery, for example, multiple battery cells assembled in a battery module.
Typically, batteries of HEVs or full electric vehicles can be cooled by air, or by a liquid coolant that, for example, has a high water content. However, when cooling fluid is provided by a pumping mechanism, such as a fan or liquid pump, in a single phase (e.g., a liquid phase), temperature gradients will exist along the flow path. That is, because heat is transferred at all points along the flow path, the temperature of the liquid coolant increases from the entrance to the exit of the battery assembly. These gradients can be somewhat reduced by increasing fluid flow rate, which consumes greater energy. Also, shortened parallel flow paths along the cells can be used instead of a serial flow path.
One example of a cooling system uses the vehicle's air-conditioning (A/C) system to cool the battery structure. However, this type of arrangement requires that considerable energy be expended to pump the refrigerant and achieve the desired cooling effects. Moreover, if the A/C system does not operate, the refrigerant will eventually all vaporize within the battery assembly, and as a consequence, the pressure and temperature inside the battery assembly will elevate. This elevation in pressure and/or temperature could result in lost battery performance due to temperature gradients that compromise the optimum operating temperature. Furthermore, the A/C system of a vehicle generally operates at temperatures around 5 degrees Celsius, which is colder than the optimum operating battery temperature range. Accordingly, a control system is required that, for example, pulses cold refrigerant into the battery assembly so as not to overcool the battery. However, this pulsing can create large and detrimental temperature gradients within the battery assembly, thus degrading the battery's life and performance.
In addition, batteries of HEV and full electric vehicles either rely on internal heat generation or heating of an adjacent fluid (e.g., air or coolant) as the main means to reach the ideal operating temperature range for their battery. As discussed above, consequences to battery life and performance at temperatures above and below that temperature range are very undesirable, and temperature uniformity among battery cells is essential to optimize performance.
Accordingly, when single phase (e.g., vapor or liquid) flow of a heated fluid is provided by some type of pumping mechanism (e.g., a fan or liquid pump) as discussed above, temperature gradients will exist along its flow path because heat is transferred at all points along the path. Therefore, the heating fluid temperature decreases from the entrance to the exit of the battery assembly, which could create temperature gradients in the battery that degrade performance. Also, internal battery heating may be a slow process, and therefore, a significant period of time colder than the ideal operating temperature may exist, which can have adverse effects such as reduce driving range as discussed above. Alternatively, multiple heaters can be used in the battery itself to attempt to evenly heat the cells of the battery. However, this arrangement would require that the temperature of each cell be monitored and the appropriate power level be applied to each heater, thus making the arrangement complicated and costly.